1. Field of the Invention
The invention relates to a fuel supply system and fuel supply method for an internal combustion engine, which vaporize liquefied fuel and supply the liquefied fuel to the internal combustion engine.
2. Description of Related Art
It is generally known that fuel that is gaseous at ordinary temperatures and pressures, such as LPG, is used as fuel for an internal combustion engine. Such gaseous fuel is cooled to be liquefied and is stored in a fuel tank. Liquefied fuel in the fuel tank is heated to be vaporized and is supplied into a cylinder of the internal combustion engine. Heat of engine coolant is generally utilized to heat and vaporize liquefied fuel; however, a fuel supply system that utilizes heat of an electric heater when the temperature of the engine coolant is low is suggested (see Japanese Patent Application Publication No. 2008-121645 (JP 2008-121645 A)).
The above described fuel supply system is configured to heat and vaporize liquefied fuel by exchanging heat between the engine coolant and the liquefied fuel when the temperature of the engine coolant is higher than or equal to a set temperature. However, when the temperature of the engine coolant is relatively high, a heat flux from the engine coolant to the liquefied fuel decreases and, therefore, the liquefied fuel may not be sufficiently vaporized.
The invention provides a fuel supply system and fuel supply method for an internal combustion engine, which suppress a decrease in heat flux from engine coolant to liquefied fuel in a heat exchanger to thereby make it possible to appropriately vaporize the liquefied fuel in a fuel tank utilizing heat of the engine coolant and to supply the vaporized fuel into a cylinder.
A first aspect of the invention provides a fuel supply system for an internal combustion engine, which supplies heated and vaporized fuel to the internal combustion engine. The fuel supply system includes a heat exchanger that has a heat exchanging wall between a liquefied fuel passage and an engine coolant passage, wherein the heated and vaporized fuel flows out from the liquefied fuel passage of the heat exchanger, a flow rate of liquefied fuel supplied to the liquefied fuel passage of the heat exchanger is set, and a flow rate of engine coolant supplied to the engine coolant passage of the heat exchanger is determined on the basis of a temperature of the engine coolant supplied to the engine coolant passage of the heat exchanger such that nucleate boiling or transition boiling of the liquefied fuel in the set flow rate occurs near a boundary between nucleate boiling and transition boiling in the liquefied fuel passage.
With the fuel supply system according to the above aspect, the fuel supply system includes the heat exchanger that has the heat exchanging wall between the liquefied fuel passage and the engine coolant passage, heated and vaporized fuel flowing out from the liquefied fuel passage of the heat exchanger is supplied to the internal combustion engine, the flow rate of liquefied fuel supplied to the liquefied fuel passage of the heat exchanger is set, and the flow rate of engine coolant supplied to the engine coolant passage of the heat exchanger is determined on the basis of the temperature of the engine coolant supplied to the engine coolant passage of the heat exchanger such that nucleate boiling or transition boiling of the liquefied fuel in the set flow rate occurs near the boundary between nucleate boiling and transition boiling in the liquefied fuel passage. In this way, when nucleate boiling or transition boiling of the liquefied fuel occurs near the boundary between nucleate boiling and transition boiling in the liquefied fuel passage of the heat exchanger, the liquefied fuel may obtain a heat flux near local maximum from the engine coolant, so the liquefied fuel may be appropriately vaporized and is supplied to the internal combustion engine.
A second aspect of the invention relates to a fuel supply method for an internal combustion engine, which supplies heated and vaporized fuel to the internal combustion engine. The fuel supply method includes: providing a heat exchanger that has a heat exchanging wall between a liquefied fuel passage and an engine coolant passage, wherein the heated and vaporized fuel flows out from the liquefied fuel passage of the heat exchanger; setting a flow rate of liquefied fuel supplied to the liquefied fuel passage of the heat exchanger; and determining a flow rate of engine coolant supplied to the engine coolant passage of the heat exchanger on the basis of a temperature of the engine coolant supplied to the engine coolant passage of the heat exchanger such that nucleate boiling or transition boiling of the liquefied fuel in the set flow rate occurs near a boundary between nucleate boiling and transition boiling in the liquefied fuel passage.